VAULT-9883: Agent Reloadable Config (#18638)

* Update command/agent.go
* Attempt to only reload log level and certs
* Mimicked 'server' test for cert reload in 'agent'

Co-authored-by: Nick Cabatoff <ncabatoff@hashicorp.com>

Left out the `c.config` tweak that meant changes to lots of lines of code within the `Run` function of Agent command. :)

command/commands.go

@@ -259,6 +259,7 @@ func initCommands(ui, serverCmdUi cli.Ui, runOpts *RunOptions) {
 					UI: serverCmdUi,
 				},
 				ShutdownCh: MakeShutdownCh(),
+				SighupCh:   MakeSighupCh(),
 			}, nil
 		},
 		"audit": func() (cli.Command, error) {